Digital processing of multi-channel signals reveals additional challenges as compared to processing single-channel signals. For example, artifacts masked in single channel coding may become audible or visible when presented as a multi-channel signal encoded as a dual mono. This relates to the difference between the masked threshold in a mono-signal presentation and the masked threshold in a multi-channel-signal presentation such as binaural listening. This effect is often referred to as the “cocktail party effect”, meaning that a person is usually able to overhear also more quiet conversations in presence of louder background noise using both ears as opposed to his/her ability with one ear plugged.
Many coding concepts of multi-channel digital signal processing aim at achieving a high coding gain while not raising the bit rate, including e.g. to dynamically allocate quantization noise to such frequency bands exhibiting amplitudes under a recognizable threshold—thus being inaudible or invisible.
In the frequency domain, the known concept of Temporal Noise Shaping (TNS) aims at further improving predictive coding techniques by enhancing the temporal resolution of a coder achieved by (adaptive prediction) TNS-filtering of the spectral coefficients of an input signal: The temporal shape of the quantization error will thus appear adapted to the temporal shape of the input signal as the quantization noise in time will be effectively localized under the actual signal, resulting in an efficient masking effect.
However, TNS filtering can also bring about disadvantages as it might increase the permissible or desired amount of side information to be transmitted to the decoder. Or, e.g. in M(id)/S(ide) stereo audio coding, quantization noise could yield audible unmasking artifacts after inverse TNS-filtering in the decoder.